It is well known that nitrogen is an essential ingredient of crop fertilizer. It may be applied to the soil in a number of different forms. One common form, widely used in the cornbelt states, is anhydrous ammonia. For economy of materials handling, due to the bulk of material involved, a practice has grown of applying anhydrous ammonia in undiluted form (82% N) in bands or ribbons beneath the soil and then covering the fertilizer to prevent its escape.
Typically, a "knife" in the form of a shank having a width of approximately 3/4 in. for forming a narrow groove having a depth of approximately 6-8 inches. A complete apparatus for applying anhydrous ammonia will normally include a number of such individual applicators or knives attached to a large frame. The frame may extend to cover a swath 12 to 40 feet in width, with individual applicator knives being typically spaced in the range of 18-30 inches. A tube is located behind the knife and extends down to provide a discharge orifice for the anhydrous ammonia in liquid form at the heel of the knife.
Various types of covering or "sealing" devices are used to fill the groove or furrow formed by the knife to prevent the escape of the ammonia gas.
One conventional sealing mechanism is a called a "paddle cover". It includes two blades spaced about 8-10 inches apart. Each blade has a general teardrop shape and is curved outwardly and thence inwardly to form converging surfaces centered on the groove formed by the applicator knife. The paddle covers gather soil from the side of the groove and scoop it laterally toward the groove to cover it. The paddle cover is effective where the soil is free from trash or debris, for example, in a field which has been plowed with a moldboard plow previously. A paddle cover does not work as well in a field where conservation tillage is practiced, and it is least effective where minimum tillage or no tillage practices are employed.
Another type of covering device is a double disc sealer which includes two curved blades (generally forming a portion of a sphere or cone) located approximately 10-12 inches apart, and again straddling the applicator groove. The axis of these blades is moved slightly upwardly from the horizontal and slightly forwardly so that the blades are at an acute angle relative to the direction of travel. The blades gather soil, lift it and throw the soil toward the opposing blade. The soil from each blade collides with the soil from the other, and this action, together with the general lifting and throwing action of the blades causes sufficient soil to be thrown in to cover the ammonia deposited at the base of the groove formed by the applicator knife.
The double disc sealer works very well in most soil conditions, but in cloddy fields or fields on which conservation tillage has been employed (leaving root crowns and large corn stalks), the blades, which are pivotally mounted and urged downwardly only under gravity in the usual course, are lifted above the surface of the ground when they engage tough trash, thereby leaving portions in which the fertilizer is not adequately covered in the groove. Twin disc sealers are more effective under most field conditions than the paddle cover, but they are also more expensive to manufacture.